Vacuum consumable arc remelting is a process used to control the solidification of segregation sensitive alloys. This control is accomplished by manipulation of system thermodynamics in a manner such that the advancing solidification region has an abundant supply of liquid metal while simultaneously minimizing local solidification time. Energy is added to the system by means of a vacuum arc and the distribution of this energy influences melt rate, fluid flow within the molten pool, and volume of the molten pool. Fluid flow is particularly affected by transient behavior of the energy distribution.
Electrode gap is one of the important variables affecting this energy distribution. As electrode gap increases, arc energy which could be used for melting may be radiated directly to the walls of the crucible in which the process is being performed and may be therefore lost to the crucible wall coolant. Electrode gap control is therefore extremely important in a successful vacuum consumable arc remelting process.
A variety of electrode gap control schemes have been attempted for use in this process. These variables include average arc voltage and the phenomena known as "hash". Average arc voltage has generally been found to be unreliable and at large arc gaps, the average arc voltage varies only a small amount in response to relatively large variations in arc gap.
A phenomena known as "hash" has also been utilized to control arc gap. Hash is a transient voltage phenomena which accompanies the transfer of metal from the electrode to the molten pool. Hash is a relatively short (10.sup.-3 second) duration increase in voltage (up to approximately 200 v) over the mean arc voltage. Applicants have discovered that this formation of "hash" or anode spikes is not particularly well correlated to electrode gap. The relationship between anode spike occurrence and electrode gap appears to be related to some unidentified experimental variable in addition to any relationship to electrode gap. While an anode spike is often associated with a drop short at electrode gaps of 0.01 to 0.035 meter, at electrode gaps greater than 0.035 meter there are slightly more anode spikes than drop shorts. It is hypothesized that the anode spikes are related to vapor starvation, a phenomenon which may occur without drop short formation at long electrode gaps. Depending upon electrode gap, the drop short may or may not be associated with an anode spike ("hash"). Thus, anode spikes or " hash" are not particularly well correlated to electrode gap.
At short electrode gaps, metal is transferred by the formation and subsequent rupture of molten metal columns (drop shorts). These columns form a low resistance bridge between the cathode (electrode) and anode (ingot). The formation of a drop short causes the arc to be momentarily extinguished resulting in a drop in the monitored voltage as measured across the electrodes. The voltage and current waveforms associated with a typical drop short are illustrated in FIG. 1 while the sketch of the occurrence of a typical drop short is illustrated in FIGS. 2 (a-d). FIG. 2(a) illustrates a molten metal column or drop short produced between the electrode and the ingot. The formation of this drop short causes the arc to be momentarily extinguished resulting in a voltage drop as measured between the cathode and anode. The voltage at point A of FIG. 1 corresponds in time to the occurrence of this drop short. Observations indicate that protuberances which develop into drop shorts grow in a cyclic manner with a frequency identical to the ripple of the power supply waveform. Protuberance growth continues until a drop short is formed or the protuberance is attacked by cathode spots. Cathode spots eject metal vapor and ions at velocities of 10.sup.3 m/s, thereby exerting a force on the molten protuberance and often driving it back into the flat electrode surface. However, when the protuberance grows into a drop short, the arc is momentarily extinguished as shown in FIG. 2a and then reignited at such a time when the drop short can no longer carry the electrical load.
At arc reignition the column is separated (usually at the anode or ingot end) and the material suspended from the electrode is supported by the impulse from accumulated cathode spots. This moment is illustrated in FIG. 2(b) and associated point B on the voltage waveform in FIG. 1. The current associated with the cathode spot accumulation results in a slow magnetic pinch (over tens of milliseconds) of the suspended column until pinch-off occurs. FIG. 2(c) and associated point C on the voltage waveform of FIG. 1 illustrate the column of metal as it is being pinched at the top of the column due to this pinching effect. At pinch-off, a globular mass of the molten metal is transferred to the ingot as shown in FIG. 2(d) and associated point D on the voltage waveform of FIG. 1. Thus, FIGS. 1 and 2 collectively illustrate the drop short phenonema and its associated voltage and current waveforms.
It is important for an understanding of the present invention to define the terms "electrode gap", "arc gap", and "arc length". These terms are not synonomous when applied to the vacuum consumable arc remelt process. Because the electrode surface is not flat during melting, but rather has several protuberances of liquid metal typically extending from it to within quite close proximity of the anode, the mean distance between electrode and ingot (electrode gap) can be much larger than the shortest distance over which an individual arc extends ("arc gap"). Further, arcs are often present at many positions on the electrode, so there are many different values of "arc length" at any given instant. Actual arc gaps of less than 0.001 meter may exist while the electrode gap is decreased to essentially zero. This is due to the support of protuberances by the impulses generated by the cathode spot phenomena. It is speculated that this is possible because cathode spot accumulation intensifies as the protuberance tip approaches the anode, causing localized deformation of the protuberance, and perhaps even of the pool surface, in effect allowing the protuberance to move below the mean surface level of the pool without making contact. When the impulse can no longer sustain the gap, contact is made and a short is observed. "Electrode gap" in this application generally refers to the mean distance between electrode and anode surfaces.